Method and apparatus for maximizing the use of available capacity in a communication system

ABSTRACT

A method and apparatus for maximizing the use of available capacity in a communication system having a base station and a plurality of mobile stations. The forward link includes a plurality of traffic streams sent on at least one channel from the base station to the mobile stations. The forward link is subject to a maximum power ceiling. A first output power level associated with simultaneously transmitting a first set of one or more traffic streams on the forward link is initially determined. Next, the first output power level is compared to the maximum power ceiling; and at least one time frame in the forward link having available capacity for transmitting a portion of at least one further traffic stream is identified. The first set of traffic streams and the portion of the at least one further traffic stream are then transmitted simultaneously during the at least one frame on the forward link.

CROSS REFERENCE

This is a divisional application of U.S. application Ser. No.09/264,435, entitled “Method and Apparatus for Maximizing the use ofAvailable Capacity in a Communication System” filed on Mar. 8, 1999, nowU.S. Pat. No. 6,317,435.

BACKGROUND OF THE INVENTION

I. Field of the Invention

This invention relates to the field of communication systems and, inparticular, to maximize the use of available capacity in a communicationsystem where signals associated with multiple users may besimultaneously transmitted on a common channel.

II. Description of the Prior Art

Telecommunications traffic can be divided into a number of classes. Oneclassification scheme divides the traffic based upon the rate at whichthe traffic is transmitted and the priority of the traffic. Inaccordance with this classification scheme, traffic is classified asconstant bit rate (CBR) traffic, variable bit rate (VBR) traffic, oravailable bit rate (ABR) traffic. (CBR) traffic is afforded a fixed bitrate regardless of the requirements of the data that is to betransmitted. This is the most expensive type of service available. VBRtraffic allows a user to decide the rate at which the traffic is sentfor each communication. ABR traffic is the lowest priority traffic. ABRtraffic is transmitted at whatever rate is available. Accordingly, ABRservice is relatively inexpensive.

One example of traffic that is best sent using CBR service isconventional fixed rate circuit switched traffic. Examples of signalshaving the variable demands suitable for VBR service are speech andInternet video services. Both CBR and VBR traffic are usually real timewith a relatively high quality of service requirement. The quality ofservice is an indication of the reliability that data will besuccessfully received as well as the delay involved in the reception.ABR traffic has a lower priority and does not provide high probabilitythat the traffic will be delivered within a short time interval. Trafficsuitable for ABR service includes file transfers and electronic mailtransfers. If loading is not high, and delay is therefore not high, mostWorld Wide Web transmissions use ABR service.

The forward link capability of a cellular communication system (i.e.,the number of users and the bit rate of each user) is in part controlledby the capabilities of the power amplifier used to amplify the signalstransmitted from the base stations of the system. For example, in a codedivision multiple access (CDMA) communication system, each of thetraffic streams transmitted is assigned to a code channel. Details of anexemplary CDMA system can be found in U.S. Pat. No. 4,901,307 entitled“Spread Spectrum Multiple Access Communication System Using Satellite OrTerrestrial Repeaters”, which is assigned to the assignee of the presentinvention and incorporated herein in its entirety by reference. Eachchannel in a CDMA system is modulated over a frequency band (which isthe same for each code channel) and combined to form a CDMA channel. Theamount of power required in each code channel depends upon the bit rateof traffic transmitted over that code channel, the gains of the antennasat the receiving station (such as a mobile station) and a transmittingstation (such as a base station), the path loss (i.e., the amount ofattenuation of the signal) between the base station and the remotestation to which the information is sent, the noise level at the mobilestation, and the performance of the modulation scheme used. The noiselevel at the mobile station includes thermal noise, noise from othercells that the mobile station is not receiving, and noise fromnon-orthogonal signal components from the cell that the mobile stationis receiving. The CDMA channel is amplified by the power amplifierwithin the base station. The base station must transmit a total powersufficient for an intended receiving mobile station to receive thesignals directed to it at the desired error rates. The base station usesvarious procedures so that the total amount of power required by theCDMA channel does not exceed the amount of power that the poweramplifier can provide without undesirable distortion.

The forward link capability of a cellular communication system is alsolimited by the amount of interference from the user's own cell (fromnon-orthogonal components if the waveform is transmitted orthogonally asin TIA/EIA-95) and by the interference from signals transmitted by othercells. This provides a limit irrespective of the amount of power thatthe base station transmits. In this situation, increasing the basestation's transmission power above some limits only marginally increasesthe capability of the system.

The maximum output power level of a base station is determined by anumber of design parameters related to the power amplifier of the basestation. Two relevant parameters of the power amplifier include powerdissipation and unwanted emissions. Unwanted emissions are emissionsthat are outside the bandwidth of a transmitted signal. A large portionof the unwanted emissions occur due to intermodulation within the poweramplifier. Intermodulation is a form of distortion. Intermodulationdistortion increases as the power amplifier is driven closer to themaximum output of the amplifier. Regulatory bodies, such as the FederalCommunication Commission often limit unwanted emissions. Industrystandards can also set limits on unwanted emissions in order to avoidinterference with the same system or another system.

In order to maintain unwanted emissions within the required limits, theoutput power capability of a power amplifier is selected to provide avery small probability that the unwanted emissions will exceed therequired limit. When the requested power exceeds the maximum outputpower, a base station can limit the output power in order to maintainthe unwanted emissions within the prescribed limits. However, the demandon the power amplifier is determined by the number of traffic streamsthat are transmitting at the same time. Each transmitted traffic streamcan start and end arbitrarily. Therefore, it is difficult to determinethe amount of power that the base station is required to transmit at anyparticular time.

An important measure in a communication system is the signal-to-noiseratio. In a digital communication system, the required signal-to-noiseratio is equal to the product of the bit rate and the required energyper bit divided by the total noise spectral density. The error rate ofthe communication system is often expressed in terms of the bit errorrate or the frame error rate. The error rate is a decreasing function ofthe signal-to-noise ratio. If the received signal-to-noise ratio is toolow, then the probability that an error will occur is very high. Thus, acommunication system attempts to maintain the received signal-to-noiseratio at or above the required signal-to-noise ratio for the desirederror rate.

Accordingly, in mobile radio communication systems such as CDMA systems,where multiple users simultaneously transmit on a common channel, thenumber of simultaneous VBR and CBR users permitted withintelecommunication system is usually limited. The limit is selected tomaintain a low probability of exceeding the maximum output power. Whenselecting the limits on the number of users, the variable rate nature ofthe VBR services and the dynamic power control on the forward link mustbe considered.

While the characteristics set forth above have been described inconnection with the forward link, similar characteristics also apply tothe reverse link.

SUMMARY OF THE INVENTION

A method for maximizing the use of available capacity in a communicationsystem (such as a CDMA system) that uses a common frequency channel forsimultaneously transmitting signals associated with multiple users isdisclosed herein. In accordance with the disclosed method, a forwardlink in a mobile radio system supports a plurality of traffic streamsassociated with multiple users and is sent on at least one commonchannel from a transmitting station (such as a base station) toreceiving stations (such as mobile stations). The forward link issubject to a maximum power ceiling. A first output power levelassociated with simultaneously transmitting a first set of trafficstreams from the base station to the mobile stations on the forward linkis initially determined. Next, the first output power level is comparedto a maximum power ceiling. At least one time frame in the forward linkhaving “available capacity” for transmitting a portion of at least onefurther traffic stream is identified. Having available capacity, meansthat the amount of power required to transmit the forward link is lowerthan the power level at which the forward link can be transmittedwithout undesirable distortion. The first set of traffic streams and theportion of the at least one further traffic stream are then transmittedsimultaneously during the at least one frame on the forward link. Thefurther traffic stream may optionally be transmitted discontinuously onthe forward link and have a lower priority than the first set of trafficstreams. Discontinuous transmission refers to the transmission overframes that are not adjacent to one another in time (i.e., frames whichdo not include the discontinuous stream are transmitted between framesthat do include the discontinuous stream).

In accordance with a preferred embodiment, any available capacity on theforward link is allocated to a second set of traffic streams in whicheach member of the second set is transmitted discontinuously on theforward link by using one or more frames. In this embodiment, a secondoutput power level is associated with simultaneously transmitting thegroup of frames from the second set of traffic streams on the forwardlink, and the sum of the first output power level (i.e., the outputpower level associated with transmitting the first set of trafficstreams on the forward link) and the second output power level is nogreater than the maximum power ceiling.

In a particularly preferred embodiment, the sum of the first and secondoutput power levels is maintained at a constant level (preferably equalto the maximum power ceiling) over a plurality of time frames. When thepresent invention is implemented in connection with a fast forward linkpower control system, the power allocation determinations necessary toimplement the invention are preferably made in a power manager locatedat a base station transceiver. Alternatively, in cases where the systemincludes a base station controller that services a plurality of basestation transceivers, the power allocation determinations may be made ina scheduler located in the base station controller and then sent to theappropriate base station transceiver.

In accordance with a further aspect, in cases in which the availablecapacity on the forward link is present over a group of one or moreframes and is allocated to a second set of traffic streams, at least oneframe in the second set of traffic streams is initially transmitted onthe forward link with a first symbol energy that is insufficient forcorrect demodulation by an intended receiving mobile station. In thisembodiment, at least one frame in the second set of traffic streamsinitially transmitted with the first symbol energy is retransmitted at alater time with a further symbol energy that may also be insufficient byitself for correct demodulation by the intended receiving mobilestation. The retransmission of the at least one frame is performed oneor more times until the sum of the symbol energy received is greatenough to permit correct demodulation by the intended receiving mobilestation.

In cases where a frame is initially transmitted with a first symbolenergy amount that is insufficient for correct demodulation by anintended receiving mobile station, that mobile station can determinethat the received frame has been received incorrectly and inform thebase station by use of a predetermined protocol. The protocol can beeither a positive or negative acknowledgement protocol. In other words,the mobile station can either send an acknowledgement when it is able tocorrectly demodulate the information or, alternatively, the mobilestation can send a negative acknowledgement; each time it is unable tocorrectly demodulate the information. Since the base station canestimate the symbol energy of the information received at the mobilestation, the mobile station may, but need not, send energy informationback to the base station when either protocol is employed. Thus, theexplicit transmission of additional energy information from the mobilestation to the base station in order to select the power level forretransmission of the frame to the mobile station is optional in thepresent invention.

In accordance with a still further aspect, the first set of trafficstreams includes at least one constant bit rate traffic stream and atleast one variable bit rate traffic stream, and frames in the constantbit rate traffic stream and frames in the second set of traffic streamsare offset in time with respect to each other. The group of frames inthe second set of traffic streams may optionally include messages thathave different lengths. In addition, each of the traffic streams mayhave a different frame length.

The aspect of the invention that initially transmits traffic informationfrom a base station with a symbol energy that is insufficient forcorrect demodulation at an intended receiving mobile station, and thenlater retransmits the same traffic information from the base stationwith additional symbol energy that is also by itself insufficient forcorrect demodulation at the intended receiving mobile station, may beapplied generally in forward or reverse link transmissions in order toachieve time diversity. In other words, this aspect of the invention maybe used to transmit any traffic stream and not simply one of thespecific traffic streams mentioned in the above embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention willbecome more apparent form the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify corresponding elements throughout and wherein:

FIG. 1 shows a graphical representation of the traffic in the forwardlink of a cellular communication system for a period covering aplurality of time frames having available capacity;

FIG. 2 shows a graphical representation of the traffic in the forwardlink of a cellular communication system for a period covering aplurality of time frames wherein all available capacity in the forwardlink has been allocated to ABR traffic;

FIG. 3 shows a graphical representation of the traffic in the forwardlink of a cellular communication system for a period covering aplurality of time frames wherein time offsets are applied totransmission signals;

FIG. 4 shows a graphical representation of the traffic in the forwardlink of a cellular communication system for a period covering aplurality of time frames wherein a predetermined scheduling policy isapplied;

FIG. 5 shows a scheduling time line of an acknowledgment protocolbetween a base station and a mobile station of a communication systemsuitable for implementation in the system of the present invention;

FIG. 6 shows a scheduling time line of a negative acknowledgmentprotocol between a base station and a mobile station of a communicationsystem suitable for implementation in the system of the presentinvention;

FIG. 7 shows a scheduling time line of a negative acknowledgmentprotocol between a base station and a mobile station of a communicationsystem suitable for implementation in the system of the presentinvention;

FIG. 8 is a block diagram showing a base station controller thatincludes a scheduler for allocating forward link power among differenttraffic streams in accordance with the present invention.

FIG. 9 is a block diagram showing two base station transceivers thateach include a power manager for allocating forward link power amongdifferent traffic streams in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a graphical representation 10 of the traffic in the forwardlink of a cellular communication system. The graphical representation 10covers a time period that includes the time frames 18A-F. The timeframes 18A-F can be, for example, twenty milliseconds in duration. Thegraphical representation 10 illustrates the use of a communicationsystem to transmit forward link traffic that includes the three constantbit rate CBR traffic streams 14A-C. All CBR traffic streams 14A-C aretransmitted during all of the time frames 18A-F. Additionally, threevariable bit rate (VBR) traffic streams 14D-F are shown in the graphicalrepresentation 10. The VBR traffic streams 14D-F alternate between onand off states and have varying transmission rates during each timeframe 18A-F.

Traffic streams 14A-F are all transmitted simultaneously on a commonchannel using for example, CDMA modulation. Within the forward link setforth by the representation 10, the time frame 18C is the most heavilyloaded because the output power required of the base station is thegreatest during the time frame 18C. More specifically, the time frame18C requires more power than the other time frames 18A-F because of therequirements of the VBR traffic streams 14D-F. The time frame 18E is themost lightly loaded because the two traffic streams 14E, 14F requirelittle power during the time frame 18E due to the relatively low bitrates. Unfilled areas 22 of the graphical representation 10 indicateunused power and therefore available capacity within the communicationsystem set forth.

FIG. 2 is a graphical representation 20 of the traffic in the forwardlink of a cellular communication system during a period covering thetime frames 18A-F. The graphical representation illustrates the use ofthe communication system to transmit traffic. The transmitted trafficincludes the three CBR traffic streams 14A-C and the three VBR trafficstreams 14D-F. The traffic streams 14A-F are transmitted as previouslydescribed with respect to the graphical representation shown in FIG. 1.Additionally, the graphical representation of FIG. 2 shows ABR trafficstreams 20A,B. It should be noted that the ABR traffic stream 20A haspriority over the ABR traffic stream 20B. ABR traffic streams 20A, B,are transmitted simultaneously on the same channel as traffic streams14A-F using, for example, CDMA modulation.

The ABR traffic streams 20A, B, use all of the remaining available basestation output power as represented by the unfilled areas 22 of thegraphical representation shown in FIG. 1. In this example, the basestation loads the forward link with CBR and VBR traffic in every timeframe 18A-F. The base station then determines which time frames 18A-Fhave additional capacity available for the transmission of the ABRtraffic by comparing the power needed for transmission of the CBR andVBR during each such frame with the maximum output power value. The basestation then schedules or transmits the ABR traffic in order to takeadvantage of the available transmit power which would otherwise remainunused. The transmission of the ABR traffic is performed consistent withthe relative priorities of each of the ABR traffic streams. Thisscheduling method is possible in the example shown in FIG. 2 because theframe lengths of the CBR, VBR, and ABR traffic are identical. It shouldbe understood that CBR or VBR streams can be used to fill in theavailable transmit power in the same manner as ABR streams provided thatthe quality of service requirements for these streams can be met.

The base station can apply different policies to determine how best toschedule or transmit the ABR traffic streams in order to take advantageof the available forward link transmit power that would otherwise remainunused. For example, after determining the power that will be requiredto transmit each of the various ABR streams buffered for transmission,the base station may simply select one or more ABR streams with powerrequirements that are likely to be equal to the available capacity.Alternatively, the base station may split the available capacity equallyamong all the ABR streams buffered for transmission. Furthermore, ABRstreams may be transmitted discontinuously. Discontinuous transmissionrefers to the transmission over frames that are not adjacent to oneanother in time (i.e., frames which do not include the discontinuousstream are transmitted between frames that do include the discontinuousstream).

As explained more fully below, in scheduling the ABR streams fortransmission, the base station may opt to transmit a given ABR stream atfull power (i.e., at the power level that the base station estimates isrequired for correct demodulation of the transmitted information at themobile station) or, alternatively, the base station may optintentionally to transmit ABR traffic information initially at less thanthe full power required for correct demodulation and then, at a latertime, retransmit the same traffic information again at less than fullpower. The mobile station receiving the multiple transmissions of thesame traffic information will then combine (or sum) both transmissionson a symbol-by-symbol basis in a buffer in order to correctly demodulatethe traffic information. In one embodiment, the base station allocatespower among a number of different streams such that none of the streamsare initially transmitted with enough power for correct demodulation bythe intended receiver. By initially transmitting the traffic informationat less than sufficient power to be correctly demodulated by an intendedreceiver and then retransmitting the same information at a later time,the base station is able to achieve time diversity in connection withthe ABR transmissions. In a fading environment, this lowers the totalrequired E_(b)/N₀. Other parameters that the base station can adjust inconnection with allocating the otherwise unused power are thetransmission rate and the code rate of the transmitted stream.

One advantage of completely filling the forward link in the mannerdescribed above is that the total power I_(or) transmitted by a basestation on the forward link is constant. Consistency in the loading ofthe forward link can simplify forward power control. However, it is notnecessary to use all of the available capacity on the forward link.Furthermore, even if all of the available capacity is used, it is notnecessary to fill the remaining power entirely with ABR trafficstream(s). For example, if there is sufficient power to allow additionalCBR or VBR traffic streams to be transmitted over the forward link, thenin one example, the available capacity can be used to transmit such aCBR or VBR traffic stream.

FIG. 3 is a graphical representation 30 of the traffic in the forwardlink of a cellular communication system during a period covering thetime frames 18A-F. The graphical representation 30 illustrates the useof a communication system to transmit traffic including the three CBRtraffic streams 14A-C and the three VBR traffic streams 14D-F. Trafficstreams 14A-C are transmitted as previously described with respect tothe graphical representations 10, 20. However, within the graphicalrepresentation 30, the frames of the VBR traffic streams 14D-F areoffset with respect to the time frames 18A-F. The frame offsets ingraphical representation 30 reduces peak processing, (i.e., the amountof information that must be processed at the same time), peak backhaulusage (the amount of information that must be communicated to otherinfrastructure components, such as base station transceivers (BTSs) andbase station controllers (BSCs)), and delay within a communicationsystem. Frame offsets of this type are well known.

In addition, the offsets shown in FIG. 3 cause the total requiredtransmit power within time frame 18A-F to vary substantially. In CDMAradio telephone systems operating in accordance with the TIA/EIA InterimStandard entitled “Mobile Station—Base Station Compatibility Standardfor Dual-Mode Wideband Spread Spectrum Cellular System”, TIA/EIA/IS-95,dated July, 1993, the contents of which are also incorporated herein byreference (the IS-95 standard), there are sixteen possible time offsetswithin a time frame 18A-F. The transmit power level can therefore varyup to sixteen times within each frame. When the transmit power levelvaries sixteen times, there is some statistical averaging of the loadbecause the number of traffic streams is large. Nevertheless, there isstill substantial variability in the transmit power level. This can makethe allocation of power for the ABR streams 20 A, B, very difficult.However, very fast power control methods are available. The powercontrol methods typically operate at eight hundred times per second perstream and therefore increase or decrease the required transmit powerper stream every 1.25 milliseconds. A system for fast forward link powercontrol is disclosed in U.S. patent application Ser. No. 08/842,993,entitled “METHOD AND APPARATUS FOR FORWARD LINK POWER CONTROL,” which isowned by the assignee of the present application and the contents ofwhich are incorporated herein by reference.

Frames 18A-F of the graphical representations 10, 20, 30 are all of thesame duration. In the preferred embodiment they are 20 ms in duration.Additionally, frames of different lengths can be used. For example,frames having a 5 ms duration that are intermixed with the frames oflength 20 ms can be used. Alternatively, frames having a longerduration, such as 40 ms can be intermixed with frames of length 20 ms.

FIG. 4 is a graphical representation 40 of the traffic in the forwardlink of a cellular communication system for a period covering the timeframes 18A-F. The graphical representation 40 illustrates a schedulingpolicy adapted to maintain the base station output power level at aconstant level. As was the case with the system shown in FIG. 2, in thesystem shown in FIG. 4, the base station schedules ABR traffic streams20A, 20B in order to take advantage of the available transmit power(i.e., blocks 22 shown in FIG. 3) which would otherwise have remainedunused. The transmit power level of the ABR traffic streams 20 A, B, canbe dynamically adjusted in order to maintain the output power constant.Thus, the base station can reduce the power of the ABR traffic streams20 A, B if it has insufficient available capacity. The adjustment can bemade in the middle of a 20 ms frame. As a result, the transmit powerlevel of the ABR traffic streams 20 A, B can be lower than required foradequate reception when using the dynamic adjustment. Similarly, thebase station can increase the power of the ABR traffic streams 20A, 20Bif the base station has available capacity. The various schedulingpolicies discussed above in connection with FIG. 2 may also be appliedin the context of the system shown in FIG. 4.

Turn now to the disclosed method mentioned above, in which the basestation intentionally transmits ABR traffic information initially withless than sufficient power required for correct demodulation by anintended receiver. Those skilled in the art will understand thatsuccessful transmission of a bit of information in a communicationsystem requires a minimum energy per bit/noise spectral density,E_(b)/N₀. The probability of a bit error is a decreasing function ofE_(b)/N₀. A frame consists of a number of bits. A frame is in error ifany of the bits in the frame is in error. In an uncoded communicationsystem, a high enough E_(b)/N₀ is required for every bit in order forthe frame not to be in error. However, in coded and interleaved systemsthe requirement does not necessarily apply to each bit. Rather, thesesystems typically require a minimum average E_(b)/N₀. The average energylevel actually required in coded and interleaved systems can depend uponthe duration of the averaging, in particular the coding andinterleaving, and the amount of energy received at various times.

Coding and interleaving are typically used to counter the effects offading that often occur in transmission channels. In communicationsystems compatible with the IS-95 standard, the coding and interleavingare performed over the duration of a 20 ms frame. Thus, in systems ofthis type the total energy received per frame is an important quantity.Therefore, it is important for understanding the relevance of thegraphical representations herein and the system and method of thepresent invention that a more detailed description of the transmissionenergy and error rates be provided.

The total energy received per frame can be represented as E_(t)/N₀. Ifthere are N coded symbols per frame, each with equal E_(s)/N₀, then:E _(t) =NE _(S) /N ₀where E_(S) is the energy of a symbol.

Let (E_(S)/N₀)_(rki) be the received E_(S)/N₀ for the ith symbol of thekth frame. Furthermore, let (E_(t)/N₀)_(rk) be the received energy inthe kth frame. Then the energy to spectral noise density received duringthe kth frame can be expressed as:

$\left( {E_{t}/N_{0}} \right)_{rk} = {\sum\limits_{i = 0}^{N - 1}{\left( {E_{s}/N_{0}} \right)_{rkr}.}}$The probability that the kth frame is correctly received (i.e., that thekth frame is received with sufficient energy to permit correctdemodulation by an intended receiver) is proportional to(E_(t)/N₀)_(rk). Thus, if (E_(t)/N₀)_(rk) exceeds a predetermined valuethere is a high probability that the kth frame is received correctly.The E_(S)/N₀ that is received at the mobile station can be determinedfrom P_(r)C/N₀/R, where P_(r) is the received power, C is the code rate,and R is the transmission rate. Alternatively, the E_(S)/N₀ can bedetermined by any one of the many techniques known to those skilled inthe art. In the case of a system such as an IS-95 system, E_(S) is theenergy per symbol received on a code channel and P_(r) is the powerreceived on the code channel.

When the transmit power of an ABR traffic stream is permitted to vary,either the bit rate or the received E_(S)/N₀ must vary. Rapid varying ofthe transmitted power of an ABR traffic stream is desired in order tomaintain a high base station output power level. However, it isdifficult to reliably signal the new transmitted rate to the mobilestation. For an IS-95 type system the output power level can changeevery 1.25 milliseconds as previously described. Thus, the receivedE_(S)/N₀ can be made to vary, and, accordingly, the (E_(t)/N₀)_(rk) canvary. The base station wastes power if it transmits at a power levelsufficient to make (E_(t)/N₀)_(rk) large enough to provide a very smallerror probability. Alternatively, if the base station transmits at apower level that is too low it can cause the error probability in theframe to be too high.

A base station can estimate the received (E_(t)/N₀)_(rk) at a mobilestation based upon the amount of power transmitted on the code channel.The base station can perform this estimation by summing the coded symbolenergies that are transmitted on the code channel. Since the total(E_(t)/N₀)_(rk) is a good indication of the probability of correct framereception, the base station can determine whether it has transmitted ahigh enough energy level to have the desired probability of correctreception. If the transmitted energy level is not high enough, the basestation can increase its transmit power level during the later parts ofthe frame in order to compensate and approach the desired transmitted(E_(t)/N₀)_(k). Likewise, if the base station transmits more energy thannecessary in the early part of the frame, it can reduce the amount ofenergy later in the frame and apply the saved energy to the remainingcode channels. The base station is not required to actually compute(E_(t)/N₀)_(rk), the base station can, instead, compute a normalizedtransmitted symbol energy value. The base station can determine therequired normalized total transmitted energy per frame using any methodknown to those skilled in the art.

As described below, the present invention can be used without theexplicit transmission of additional energy information from the mobilestation to the base station. In particular, the mobile station candetermine whether the received frame is received correctly or not andperform an acknowledgment protocol with the base station. The protocolcan be either a positive or negative acknowledgement protocol. In otherwords, the mobile station can either send an acknowledgement when it isable to correctly demodulate the information or, alternatively, themobile station can send a negative acknowledgement each time it isunable to correctly demodulate the information. Two exemplaryacknowledgment protocols that may be used in connection with the presentinvention are discussed below in connection with FIGS. 5 and 6. If pastpower control is being used, the base station can estimate the symbolenergy of the information received at the mobile station. Then themobile station may, but need not, send energy information back to thebase station when either protocol is employed. Thus, the transmission ofsuch energy information from the mobile station back to the base isoptional in the present invention.

Dynamically varying the amount of transmitted power can adversely affectthe demodulation process in the mobile station receiver. In thereceiver, the optimal process is weighting the accumulated symbolamplitude by the signal to noise ratio for each symbol. Such a weightingprocess is described in U.S. Pat. No. 6,101,168, entitled “METHOD ANDAPPARATUS FOR TIME EFFICIENT RETRANSMISSION USING SYMBOL ACCUMULATION,”which is owned by the assignee of the present invention and the contentsof which are hereby incorporated herein by reference. In most IS-95implementations, the weighting uses the common pilot signal because thecode channel power is constant over a frame and the pilot E_(c)/I₀ is ascaled value of the signal to noise ratio. With fast forward link powercontrol (as described in U.S. patent application Ser. No. 08/842,993cited above), the power can be varied in a frame so that the power of acode channel is not in constant proportion to the common pilot signal.Power variations within a frame are not a problem because the mobilestation can develop an appropriate weighting if necessary. However, whenthe base station reduces the transmitted energy of a code channel inorder to use it on one or more other code channels, the weighting can bevery different and the mobile station may not be aware of the power thatthe base station is using. For example, the weighting applied to the ABRstream 14F of the graphical representation 30 at the end of the firstframe can be much greater than that applied at the end of the thirdframe. It will be understood that a large amount of power is transmittedfor the stream at the end of the first frame and that little power istransmitted at the end of the third frame. For an accurate weighting insuch situations, the mobile station can estimate the energy and noise inthe received symbols and apply the appropriate weighting.

Rather than using the common pilot channel for weighting as described inthe paragraph above, it is also possible to develop the weighting usinga dedicated pilot channel. A dedicated pilot channel is a pilot that isdirected to a specific mobile station. The dedicated pilot power wouldbe part of the power that is being transmitted to the specific mobilestation. With the dedicated pilot, it may be possible to adjust thepilot level in proportion to the transmitted power on the data channel.A drawback of this approach is that it has the impact of increasing thevariance of the phase estimator, thus degrading the performance.Moreover, the dedicated pilot channel approach for weighting may notwork if there are non-ABR services being transmitted to the mobilestation, and such non-ABR services require a high pilot level for properperformance. In such cases, the level of the dedicated pilot will bemaintained at a high level, thereby wasting power and precluding the useof the dedicated pilot channel for development of the weighting.

Under the above conditions the mobile station may not receive the ABRtraffic stream with sufficient power to demodulate the stream with fewenough errors (i.e., to demodulate the stream correctly). The mobilestation can use a combination of checking the cyclic redundancy check(CRC) bits, testing the re-encoded symbol error rate, and checking thetotal received energy in order to determine whether the frame issignificantly erred. Other techniques known by those skilled in the artcan also be used.

In accordance with the present invention, when a frame is determined tobe in error, the mobile station stores the received code symbols for theframe in a buffer. In accordance with one embodiment of the presentinvention, the mobile station then computes (E_(t)/N₀)_(k) based uponthe energy received in the frame. The amount of additional(E_(t)/N₀)_(k) required for the frame to be demodulated with therequired error rate can then be estimated. The mobile station sends tothe base station a negative acknowledgment and may include such anestimate of the amount of additional (E_(t)/N₀)_(rk) required. The totalrequired (E_(t)/N₀)_(k) can be estimated in this power control methodbased upon the outer loop power required (or the threshold) for thefundamental channel or DCCH channel. U.S. patent application Ser. No.08/842,993 (cited above) discloses a method for estimating the totalrequired (E_(t)/N₀)_(k) based upon the outer loop power required.Alternately, there can be a separate outer loop power control method forthe channel being used. It will be understood that if the frame isreceived incorrectly (i.e., with an undesirable number of errors), then(E_(t)/N₀)_(k) is insufficient. Thus, the optimum power level can bedetermined by conditional statistics that take into account the factthat previous attempts were received incorrectly. Instead of sending theamount of additional (E_(t)/N₀)_(rk) that is required, the mobilestation can send the amount of (E_(t)/N₀)_(rk) that was received to thebase station. The mobile can also include an estimate of the amount thatit expects to need for correct demodulation in information sent to thebase station.

FIG. 5 is a graphical representation 50 showing a scheduling time lineof an acknowledgment protocol between a base station and a mobilestation of a communication system suitable for implementation of themethod of the present invention. The acknowledgement protocol of thegraphical representation 50 can be used in a power control method as setforth above.

A preferred embodiment of the method of the graphical representation 50can be implemented in an IS-95 third generation system. In the IS-95third generation system a supplemental channel (F-SCH) can be used fortransmission of the ABR traffic streams on the forward link. Thesupplemental channel is typically a scheduled channel, though it canalso be a fixed or a variable rate channel. The F-DCCH and R-DCCH areforward and reverse control channels respectively. When the supplementalchannel (F-SCH) is used for transmission of the ABR traffic streams onthe forward link in accordance with the present invention, the errorrate of the DCCH channels is typically lower than that of thesupplemental channel (F-SCH). In the acknowledgement protocol ofgraphical representation 50, the base station transmits the schedule inmedium access control (MAC) messages 94 and 98 to the mobile station.The schedule informs the mobile station of a number of aspects of thetransmissions, which can include, but are not limited to, the number offrames that will be transmitted, their transmission rates, when theywill be transmitted, and their frame numbers. In one embodiment of theinvention, the MAC message 94 only provides the mobile station with thetransmission rate that will be used. With this embodiment, the mobilestation continually attempts to receive the F-SCH.

The base station indicates that two radio link protocol (RLP) frames102, 104 must be sent to the mobile station. RLP is the upper layerframing protocol of the communication system. An RLP similar to thatdescribed in TIA standard IS-707 can be used, though many differentupper layer framing protocols can be used. In what follows, an RLP frameis assumed to map exactly to a physical layer frame though that is notnecessary as part of this invention. The sequence numbers of the RLPframes 102, 104 are K and K+1, respectively. The RLP frames 102, 104 aretransmitted during the physical frames I+1 and I+2, respectively. Whenthe mobile station correctly receives the transmission of the RLP frameK+1 (104), it acknowledges the frame using message 112. Since the basestation does not receive an acknowledgement of the RLP frame K (102),the base station sends a new forward link assignment in the MAC message98 indicating that the RLP frame K is scheduled for retransmissionduring the physical frame I+5 (110). The mobile station learns from theMAC message 98 that it must combine the signal received during frame I+5(110) with the signal received during frame I+1(102). After physicalframe I+1 is retransmitted during the physical frame I+5, the mobilestation combines the received energy for each symbol in theretransmitted physical frame I+5 with the received energy of theoriginal transmission during frame I+1 (stored in the buffer asdescribed above) and decodes the combined received energy of the framesas described herein.

The mobile station acknowledges the RLP frame K during the frame I+6using the acknowledgement message 114. With this acknowledgement basedmethod, the energy deficit is not transmitted to the base station.Moreover, in further embodiments, the energy deficit may be sent to thebase station with the acknowledgement of the RLP frame K+2. Thus, inthis embodiment, the acknowledgement always carries the estimate of theamount of additional (E_(t)/N₀)_(k) required from the first frame thatwas in error. However, this method may not work well if the last framein a sequence of frames is not received correctly by the mobile station.

When the base station determines that an acknowledgement was notreceived from the mobile station and it desires to retransmit themessage, the base station determines the level at which to transmit themessage. The base station can choose a level based upon the feedbackinformation on the amount of required energy needed by the mobilestation. Alternatively, the base station can estimate the amount ofenergy that the mobile station has already received and use this todetermine the level at which to re-transmit. The power level chosen forretransmission will, in one embodiment, correspond to a minimum powerlevel needed for correct demodulation when the symbol energy of theoriginal message and the retransmitted messages are combined in thereceiver buffer. The base station can form an estimate of the amount ofenergy that the mobile station has already received using informationfrom forward power control, the transmission rate, the propagationconditions, the amount of power already used to transmit the frame, andthe path loss. The actual information used in developing this estimatecan include these or any other parameters which are available to thebase station. Alternatively, the base station can just transmit a fixedpower (or fixed power relative to the forward power control level) tothe mobile station. This fixed power level could have been predeterminedby the base station.

Instead of the explicit method of the base station transmitting message98 to the mobile station to provide the identity of a retransmittedframe, the mobile station can alternatively implicitly determine theidentity of the retransmitted frame with a reasonably degree of accuracyfrom the transmitted data. For example, the Euclidean distance can beused to determine whether frame I+5 matches the data received inprevious frames that have not been acknowledged, such as frame I+1.Thus, the explicit retransmission of message 98 is not required for thisinvention. In this alternative embodiment, the mobile station comparesthe received symbols from the current frame with symbols from allprevious frames stored in the mobile station's buffer. If the mobilestation determines that the retransmitted frame corresponds to a framealready within the buffer, the mobile station combines the energies foreach symbol and attempts to decode the frame.

In an alternative embodiment of the protocol shown in FIG. 5, message 94is not required. Message 94 is used in the embodiment described above toprovide an indication to the mobile station that frames 102 and 104 areto be transmitted. In this alternative embodiment, the mobile stationcan alternatively determine implicitly whether the current frame is anew frame or a retransmitted frame with a reasonably degree of accuracyfrom the transmitted data using the Euclidean distance analysisdescribed previously.

FIG. 6 is a graphical representation 60 showing a scheduling time lineof a negative acknowledgement protocol between a base station and amobile station suitable for implementation in the system of the presentinvention. The negative acknowledgement protocol of the graphicalrepresentation 60 can be used in a power control method as set forthabove.

In the negative acknowledgement protocol of graphical representation 60,the base station informs the mobile station of the RLP frames 102, 104to be transmitted and the physical layer frames to be transmitted bymeans of the MAC message 94. The base station then sends the frames 102,104 to the mobile station. If the mobile station does not receive theRLP frame 102 correctly, the mobile station sends a negativeacknowledgement 116 to the base station. The base station then sendsmessage 98 as previously described and the information of the frame 102is retransmitted as the frame 110.

One of the disadvantages of the negative acknowledgement based protocolis that the base station is not able to take action to retransmit frame102 if the negative acknowledgement is not received from the mobilestation. For ABR traffic, the probability that a frame transmitted onthe forward link is in error is much greater than the probability thatthe negative acknowledgement sent on the reverse link is in error. Thisis because the amount of power required to transmit a frame with manybits on the forward link is considerably higher than the amount of powerrequired to transmit an acknowledgment. The negative acknowledgmentprotocol can use a MAC message 98 to indicate that the frame is beingretransmitted. The MAC message 98 can be similar to that used for theacknowledgement protocol shown in FIG. 5. The negative acknowledgementprotocols can also use an implicit method for determining the identityof a retransmitted frame that is similar to that described for theacknowledgment protocol shown in FIG. 5.

Several alternate embodiments of the negative acknowledgement basedprotocol are possible. In one alternate embodiment, the base stationdoes not inform the mobile station about the frames of the originaltransmission and does inform the mobile station of time intervalswherein the frames can be sent. The mobile station demodulates all ofthe physical frames. If the mobile station correctly receives the RLPframe K+1, it transmits a negative acknowledgement for the missingframes (which includes the KTH frame) on the R-DCCH. A disadvantage ofthis protocol is that the mobile station does not know when to releasememory used to store the symbol energies from the various frames. Thisdisadvantage can be addressed in several ways. One way is providing afixed amount of memory and having the mobile station discard the oldestreceived physical layer frame symbol energies when it needs additionalmemory. Alternatively, the mobile station can discard memorycorresponding to a physical layer frame that was received more than apredetermined time in the past.

A further disadvantage of this protocol is that the mobile station maynot have information about when to send a negative acknowledgementpromptly for frames that are received in error. This disadvantage iscompounded by the fact that only a few frames may be received correctlyon the first transmission. This disadvantage can be overcome if the basestation occasionally transmits a second done message to the mobilestation on the F-DCCH. This done message informs the mobile station thatthe base station has transmitted a sequence of frames, thus permittingthe mobile station to determine the frames which it should havereceived. The mobile station can then send a negative acknowledgemessage for the frames that it did not receive. Any done message can becombined with any other message, such as a message that indicates thatthe frames will be transmitted.

Significantly, when a frame is initially transmitted with insufficientenergy to permit correct demodulation by the intended receiver, asdescribed above, and then retransmitted, the retransmission providestime diversity. As a result, the total transmit energy of the frame(including retransmissions) is lower. In other words, the combinedsymbol energy for both the initial transmission and retransmission(s) ofthe frame is lower than the energy that would have been required totransmit the frame initially at full power (i.e., at a power level thatwas sufficient on its own to permit correct demodulation by the intendedreceiver). This can be determined because the required E_(b)/N_(t) for apredetermined bit error rate or frame error rate is lower when thismethod of retransmission is used.

Furthermore, it will be understood that the fast forward link powercontrol (as described in U.S. patent application Ser. No. 08/842,993cited above) is less important in the case of ABR traffic streams thatutilize the retransmission approach described above. The fast forwardlink power control is less important because the retransmission approachis a form of power control. In addition, fast forward link power controlmay be less important when the retransmission approach is beingemployed, because fast forward link power control attempts to maintainthe E_(b)/N_(t) constant at the mobile station. Thus, it may bepreferable to not use fast forward power control for ABR services.

In the case of the forward link, the base station adjusts its transmitpower to the channel when it is unable to supply additional power forthe channel from the base station. This can occur, for example, when aVBR user or a set of VBR users, a higher priority stream a (CBR or VBRstream), or a set of high priority streams require more transmit powerdue to different path losses or propagation conditions, or when theforward link path loss increases between the mobile unit and the basestation.

The present invention has been described above with respect tovariations in base station loading for transmitting forward linkservices such as CBR and VBR streams and variations due to powercontrol. However, it will be understood that the invention can beadvantageously applied to other situations including transmissions onthe reverse link.

In the case of the reverse link, an important parameter is the rise inthe level of the total amount of noise over the level of the thermalnoise at a base station (referred to hereafter as the “rise overthermal”). The rise over thermal corresponds to the reverse linkloading. A loaded system attempts to maintain the rise over thermal neara predetermined value. If the rise over thermal is too great the rangeof the cell is reduced and the reverse link is less stable. A large riseover thermal also causes small changes in instantaneous loading thatresult in large excursions in the output power of the mobile station.However, a low rise over thermal can indicate that the reverse link isnot heavily loaded, thus potentially wasting available capacity. It willbe understood by those skilled in the art that methods other thanmeasuring the rise over thermal can be used to determine the loading ofthe reverse link.

ABR traffic streams can also be allocated available capacity on thereverse link to keep the rise over thermal more constant. The basestation can control the reverse link transmission with a form of highrate RLP control. The third generation of IS-95 has a single powercontrol stream that controls the pilot, the R-FCH, the R-SCH, and theR-DCCH simultaneously. Slower signaling is used in this IS-95 embodimentto control the power allocation between the channels. Typically theR-SCH requires most of the transmit power since it is carrying the highrate data stream. If all channels are controlled by the high rate powercontrol stream, then when the base station requires a reduction of thepower on the R-SCH in order to control loading, the power of allchannels is reduced. This is not desirable because the pilot, the R-FCH,and the R-DCCH can be received by the base station at a level that istoo low.

A separate high rate power control channel from the base station to themobile station can be used for reverse link power control on an IS-95third generation system. The power control rate for the reverse link canbe eight hundred bits per second. While the same rate can be used tocontrol the R-SCH independently of the other channels, the 800 bps raterequires more base station transmission power than necessary. Thus, thepower control rate for the R-SCH can be somewhat lower because it doesnot have to be maintained perfectly in fading conditions. Furthermore,the power control for the R-SCH can be at an offset with respect to themain power control stream that controls the R-SCH, the R-DCCH, and thepilot. A signaling message or other signaling scheme can be transmittedto the mobile station to provide this relative power control in lieu ofa power control bit stream.

In an alternative embodiment, a separate low rate power control streamcan be used to provide a correction to all mobile stations relative totheir own individual power control streams. This can be a binary streamspecifying an increase or decrease in power for mobile stations relativeto their own individual power control streams. This can also be a threelevel method that can indicate increase, decrease or do not change.Additionally, any other known power control scheme can be used for theseparate low rate power control.

The disclosed method can also be used when a mobile station hasinsufficient power to transmit all of the streams to be transmitted toan intended receiver at a receive power level that allows correctdemodulation. In such a case, the mobile station can reduce thetransmitted power on the R-SCH to attempt to maintain the R-FCH andR-DCCH at the desired output power level. This method is similar to amethod used on the forward link. Since the base station will receivesome power from the mobile station, the amount of power required duringthe retransmission will be less.

FIG. 7 shows a graphical representation 70. The graphical representation150 sets forth a scheduling time line of a negative acknowledgementprotocol on a reverse link between a base station and a mobile stationof a communication system suitable for use with the present invention.The negative acknowledgement protocol of the graphical representation 70can be used in a power control method as set forth above.

Most of the timing and the acknowledgement structure of the reverse linkoperates in the same manner as described with respect to the forwardlink. An exception is the following. In the reverse link, the mobilestation requests permission to transmit the high rate ABR frames 164,168 by means of the request 176. The base station informs the mobilestation when to send ABR frames 164, 168 by means of an assignmentmessage 152. The mobile station of the graphical representation 70 isnot required to request retransmission of an erred frame 164. Howeverthe base station knows that the frame 164 is in error and schedules aretransmission when the reverse link has available capacity.Furthermore, a negative acknowledgement message 156 transmitted by thebase station can include permission to retransmit a reverse link powerframe 172 and the slot in which it is transmitted.

The alternative embodiments previously described above with respect tothe forward link can also be applied to the reverse link. For example,in one embodiment of the reverse link, the mobile station is notrequired to request transmissions using the MAC message 176.Furthermore, the base station is not required to grant access to thechannel using the MAC messages 152. In another embodiment, the basestation is not required to explicitly inform the mobile station usingmessage 176 of the frame in which to retransmit the message.

Referring now to FIG. 8, there is a block diagram 80 showing a basestation controller (BSC) 810 that includes a scheduler 812 forallocating forward link power among different traffic streams inaccordance with one embodiment of the present invention. The variouspolicies for allocating power to the ABR transmission streams may beimplemented in software using scheduler 812. Operation of a schedulerthat may be modified to include software for allocating power inaccordance with the present invention is disclosed in U.S. patentapplication Ser. No. 08/798,951 entitled “NEW AND IMPROVED METHOD FORFORWARD LINK RATE SCHEDULING,” which is owned by the assignee of thepresent invention and the contents of which are hereby incorporatedherein by reference. In the embodiment shown in FIG. 8, BSC 800determines the power allocation for each of the data streams beingtransmitted; this power allocation information is then transmitted tobase station transceiver systems (BTSs) 820, 822, which in turn transmitthe various data streams to one or more mobile stations 830 inaccordance with the power allocation determinations made at scheduler810.

Referring now to FIG. 9, there is a block diagram 90 showing two basestation transceivers 820A, 822A that each include a power manager 821for allocating forward link power among different traffic streams inaccordance with an alternative embodiment of the present invention. Theembodiment shown in FIG. 9 is useful in cases where fast forward powercontrol is being applied, because in this embodiment the powerallocation determinations are made at the BTS's (rather than at the BSC800), thereby eliminating the delay resulting from transmission of thepowers being transmitted on the forward link from the BTSs to the BSCand the power allocation information from BSC 800 to the BTSs. In theembodiment shown in FIG. 9, the various policies for allocating power tothe ABR transmission streams may be implemented in software using powermanagers 821. Each power manager 821 determines the power allocation foreach of the data streams being transmitting by the corresponding BTS,and the BTS then transmits the various data streams to one or moremobile stations 830 in accordance with the power allocationdeterminations made by power manager 821. In another embodiment, thescheduler 810 in the BSC can set some general power allocation policythat the power managers 821 in the BTSs carry out. This has theadvantage that the power managers 821 can handle short term fluctuationswithout encountering the delay between the BTS and the BSC and providesa consistent scheduling policy over all data streams.

In summary, different scheduling policies are possible during thetransmissions of time frames 18A-F. A frame scheduling policy is a setof rules for determining which of a plurality of signals waiting to betransmitted are actually inserted into a frame. In one schedulingpolicy, a base station can transmit the traffic streams that are likelyto be received at sufficient power by the intended receiving mobilestation. Alternately, a scheduling policy can be used wherein theforward link is transmitted with sufficient power for correctdemodulation by the intended receiving mobile station on the firsttransmission. In an alternate embodiment, the base station can allocatepower to a number of different streams such that none of the streams aretransmitted with enough power to allow reliable decoding by the receiverwithout at least one retransmission, as previously described. Thetransmission rate and the code rate of the transmitted stream are amongthe other parameters that the base station can adjust in this case.Furthermore, one embodiment of the invention is directed to the casewherein a mobile station has insufficient power to transmit all of thebit streams. In this case, the mobile station can reduce the transmittedpower on the R-SCH in an attempt to maintain the R-FCH and R-DCCH at therequired power level. This method is similar to the one used for theforward link. Since the base station receives some power from the mobilestation, the amount of power required during the retransmission is less.It will be understood that all of the methods disclosed herein can beused at the time of call set up or at any time during a transmissionafter set up.

The previous description of the preferred embodiments is provided toenable a person skilled in the art to make or use the present invention.The various modifications to these embodiments will be readily apparentto those skilled in the art, and the generic principles defined hereincan be applied to other embodiments without the use of the inventivefaculty. Thus, the present invention is not intended to be limited tothe embodiments shown herein but is to be accorded the widest scopeconsistent with the principles and novel features disclosed. It shouldbe further noted that the paragraphs and subparagraphs within the claimsare identified with letter and number designations. These designationsdo not indicate the order of importance of the associated limitations orthe sequential order in which steps should be performed.

1. In a radio communication system having a base station and a pluralityof mobile stations, a method for transmitting traffic information fromthe base station to a mobile station, comprising the steps of: (A)intentionally transmitting the traffic information from the base stationwith a first symbol energy amount that is intentionally insufficient forcorrect demodulation of the traffic information by the mobile station;(B) after step (A), retransmitting from the base station the trafficinformation initially transmitted with the first symbol energy amount,wherein the traffic information is retransmitted in step (B) with afurther symbol energy amount that is also insufficient by itself forcorrect demodulation of the traffic information by the mobile station;and (C) repeating step (B) until a sum determined at the mobile stationof the first and further symbol energy amounts used to transmit thetraffic information is great enough to permit correct demodulation bythe mobile station.
 2. The method of claim 1, wherein the further symbolenergy amount used for re-transmitting the traffic information in step(B) is determined at the base station using fast forward power control.3. The method of claim 1, further comprising the steps of: (D)determining, at the mobile station, a received energy valuecorresponding to the traffic information transmitted from the basestation in step (A); and (E) transmitting the received energy value fromthe mobile station to the base station; (F) wherein the further symbolenergy amount used for re-transmitting the traffic information in step(B) is determined at the base station in accordance with the receivedenergy value transmitted from the mobile station.
 4. The method of claim3, wherein the received energy value is transmitted from the mobilestation to the base station using an acknowledgement protocol.
 5. Themethod of claim 4, wherein the acknowledgement protocol is transmittedbetween the base station and the mobile station using forward andreverse control channels.
 6. The method of claim 5, wherein the trafficinformation is transmitted in steps (A) and (B) on a supplementalchannel, and the forward and reverse control channels have a lower errorrate than the supplemental channel.
 7. The method of claim 3, whereinthe received energy value is transmitted from the mobile station to thebase station using a negative acknowledgement protocol.
 8. The method ofclaim 7, wherein an acknowledgement protocol is transmitted between thebase station and the mobile station using forward and reverse controlchannels.
 9. The method of claim 8, wherein the traffic information istransmitted in steps (A) and (B) on a supplemental channel, and theforward and reverse control channels have a lower error rate than thesupplemental channel.
 10. The method of claim 1, further comprising thestep of: (D) summing the traffic information transmitted with the firstsymbol energy amount in step (A) with the traffic informationtransmitted with the further symbol energy amount in step (B) bycombining received energy associated with the traffic informationtransmitted with the first symbol energy amount in step (A) withreceived energy associated with the traffic information transmitted withthe further symbol energy amount in step (B) in a buffer at the mobilestation; and (E) demodulating the traffic information at the mobilestation in accordance with the result of step (D).
 11. In a mobile radiotelephone system having a base station controller that services aplurality of base station transceivers that transmit traffic informationto a plurality of mobile stations, an apparatus for transmitting trafficinformation from a base station transceiver to a mobile station,comprising: (A) a power allocation unit at the base station controllerthat selects a first symbol energy amount for transmitting the trafficinformation from the base station transceiver to the mobile station,wherein the power allocation unit allocates power among a number ofdifferent streams of the traffic information such that none of thestreams are initially transmitted with enough power for correctdemodulation by the mobile station, and the power allocation unitselects a further symbol energy amount for retransmitting the trafficinformation from the base station transceiver to the mobile station,wherein the further symbol energy amount is also insufficient by itselffor correct demodulation of the traffic information by the mobilestation; (B) a base station transmitter that initially transmits thetraffic information from the base station transceiver to the mobilestation at the first symbol energy amount and subsequently retransmitsthe traffic information from the base station transceiver to the mobilestation at the further symbol energy amount; and (C) a buffer in themobile station that combines the retransmitted traffic information fromeach of the streams of the traffic information until a sum determined atthe mobile station of the first and further symbol energy amounts usedto transmit the traffic information is great enough to permit correctdemodulation of the streams by the mobile station.
 12. In a mobile radiotelephone system having a base station that transmits trafficinformation to a plurality of mobile stations, an apparatus fortransmitting traffic information from the base station to the mobilestation, comprising: (A) a power allocation unit at the base stationthat selects a first symbol energy amount for transmitting the trafficinformation from the base station to the mobile station, wherein thefirst symbol energy amount is insufficient for correct demodulation ofthe traffic information by the mobile station, and the power allocationunit selects a further symbol energy amount for retransmitting thetraffic information from the base station to the mobile station, whereinthe further symbol energy amount is also insufficient by itself forcorrect demodulation of the traffic information by the mobile station;(B) a base station transmitter that initially transmits the trafficinformation from the base station to the mobile station at the firstsymbol energy amount and subsequently retransmits the trafficinformation from the base station to the mobile station at the furthersymbol energy amount; and (C) a buffer in the mobile station thatcombines retransmitted the traffic information until a sum determined atthe mobile station of the first and further symbol energy amounts usedto transmit the traffic information is great enough to permit correctdemodulation.
 13. In a mobile radio telephone system having a basestation and a plurality of mobile stations, an apparatus fortransmitting traffic information from the base station to a mobilestation, comprising: (A) means for intentionally transmitting thetraffic information from the base station with a first symbol energyamount that is insufficient for correct demodulation of the trafficinformation by the mobile station; (B) means for re-transmitting fromthe base station the traffic information initially transmitted with thefirst symbol energy amount, wherein the traffic information isretransmitted with a further symbol energy amount that is alsoinsufficient by itself for correct demodulation of the trafficinformation by the mobile station; and (C) means for repeating step (B)until a sum determined at the mobile station of the first and furthersymbol energy amounts used to transmit the traffic information is greatenough to permit correct demodulation by the mobile station.